The present disclosure relates to a piston skirt coating and to a method for applying such a coating.
To ensure running-in and fail-safe operating properties, piston skirt coatings are known having a structure of functionally graduated structure, in the composition of which a compromise must be found between friction-reducing and wear-resistant properties.
Thus, especially wear-resistant pistons have carbon fiber-reinforced coatings with a high roughness value, which brings about disadvantages with respect to process monitoring (increased difficulty in determining film thickness) and the running-in phase of the engine (longer running-in period, increased friction). The hard fibers have an abrasive effect on cylinder walls which can result in undesirable polishing effects, or to the formation of grooves.
It is known from US 2009/0156437 A1 to furnish pistons for internal combustion engines with a double layer of an inner polymer layer having a solid lubricant and an outer polymer layer with hard particles of SIN or Al2O3. The outer hard coating is intended to reduce running-in time for the piston and to improve seize characteristics.
EP 1 433 838 A1 similarly describes a coating for moving metal parts that consists of an inner polymer layer with dispersed hard particles of Al2O3 and silicon nitride and an outer layer also with dispersed hard particles and solid lubricant. The hard particles in the outer layer are of a smaller diameter as those in the inner layer, which is intended to reduce the abrasiveness of the coating.
EP 1 933 022 A2 discloses a low-friction piston skirt coating made from a polymer layer containing a mixture of hard carbon or aramid fibers and solid lubricants.
All the coatings described here have a severe abrasive effect on account of the hard and partially rough outer layer, which requires large tolerances during manufacture and limits the use of the pistons with respect to the material of the cylinder wall. Wear-resistant outer layers lead, in particular with aluminum cylinder walls, to undesirable polishing effects or the formation of grooves, which, with increased friction, results in longer running-in time during the running-phase of the engine.
In contrast, U.S. Pat. No. 5,884,600 proposes an inner, hard coating of aluminum oxide for aluminum pistons, produced by anodizing the piston surface and an outer polymer layer containing solid lubricants such as graphite and boron nitride. The application of the first, hard aluminum oxide layer by anodizing the piston surface is a costly process, difficult to control regarding film thickness of the layer and results in a large number of unusable pistons in series production.
With respect to aluminum cylinder walls, iron-coated aluminum pistons have usually been used heretofore, that ensure increased protection against seizing and additionally do not affect cylinder honing. Iron coating represents a cost-intensive process in comparison to polymer-based coatings. In addition, especially wear-resistant layers usually contribute to greater friction because the adaptability of the layers to their opposite number is low.
It would be desirable to provide coatings for pistons for internal combustion engines that can be applied simply, that are wear-resistant and have low frictional losses as the result of improved running-in characteristics.